In a differentially pumped mass spectrometer system a sample and carrier gas are introduced to a mass analyser for analysis. One such example is given in FIG. 1. With reference to FIG. 1, in such a system there exists a high vacuum chamber 10 immediately following first and second evacuated interface chambers 12, 14. The first interface chamber 12 is the highest-pressure chamber in the evacuated spectrometer system and may contain an orifice or capillary through which ions are drawn from the ion source into the first interface chamber 12. The second, interface chamber 14 may include ion optics for guiding ions from the first interface chamber 12 into the high vacuum chamber 10. In this example, in use, the first interface chamber 12 is at a pressure of around 1 mbar, the second interface chamber 14 is at a pressure of around 10−2 to 10−3 mbar, and the high vacuum chamber 10 is at a pressure of around 10−5 mbar.
The high vacuum chamber 10 and second interface chamber 14 can be evacuated by means of a compound vacuum pump 16. In this example, the vacuum pump has a first pumping section 18 and a second pumping section 20 each in the form of a set of turbo-molecular stages, and a third pumping section in the form of a Holweck drag mechanism 22; an alternative form of drag mechanism, such as a Siegbahn or Gaede mechanism, could be used instead. Each set of turbo-molecular stages comprises a number (three shown in FIG. 1, although any suitable number could be provided) of rotor 19a, 21a and stator 19b, 21b blade pairs of known angled construction. The Holweck mechanism 22 includes a number (two shown in FIG. 1 although any suitable number could be provided) of rotating cylinders 23a and corresponding annular stators 23b and helical channels in a manner known per se.
In this example, a first pump inlet 24 is connected to the high vacuum chamber 10, and fluid pumped through the inlet 24 passes through both sets of turbo-molecular stages in sequence and the Holweck mechanism 22 and exits the pump via outlet 30. A second pump inlet 26 is connected to the second interface chamber 14, and fluid pumped through the inlet 26 passes through one set of turbo-molecular stages and the Holweck mechanism 22 and exits the pump via outlet 30. In this example, the first interface chamber 12 may be connected to a backing pump (not shown), which may also pump fluid from the outlet 30 of the compound vacuum pump 16. As fluid entering each pump inlet passes through a respective different number of stages before exiting from the pump, the pump 16 is able to provide the required vacuum levels in the chambers 10, 14.
In some such applications, a Holweck mechanism such as that illustrated in FIG. 1 typically provides a backing pressure to the second pumping section 20 of around 0.01 mbar to 0.1 mbar. The use of turbomolecular stages for a pumping section having such a relatively high backing pressure to produce an inlet pressure of above 10−3 mbar may cause excessive heat generation within the pump and severe performance loss, and may even be detrimental to the pump reliability. In view of this, our co-pending International patent application PCT/GB2004/004114, the contents of which are hereby incorporated by reference, describes a compound vacuum pump in which the second pumping section 20 is provided by an externally threaded, or helical, rotor. Such a compound vacuum pump 40 is illustrated in FIG. 2, in which the helical rotor is indicated at 42. In such a pump, the inlet of the helix of the helical rotor will behave in use like a rotor of a turbo-molecular stage, and thus provide a pumping action through both axial and radial interactions. As discussed in the above-referenced co-pending application, an advantage of the use of such a deep groove helical rotor in place of the set of turbomolecular stages is that it can offer a comparable pumping capacity, but with lower levels of power consumption and heat generation.
It is an aim of at least the preferred embodiment of the present invention to further improve the performance of a differential pumping, multi port, compound vacuum pump that includes a pumping section comprising a helical rotor.